"Import Issues from CSV" (Issues > Import issues from CSV) skip importing fields absent on the screen.

BUT

If we import the same CSV from (Jira administration > System > External System Import > CSV), then we are able to import the field fine even if the field is absent on the issue create screen.

Observation:

Another interesting observation is, even if the field is absent on the issue create screen, the field is available to map in the dropdown when we are importing the csv (could be because of the field context) and the validator doesn't throw any warning if it is missing the field. This is in the case of (Issues > Import issues from CSV) functionality.
If we are skipping importing such fields, there needs to have improvement there as well and it should call out a field name if it is not going to import it.

One can find if the data is imported fine in the absent field, by going to the list view of the screen and make that field available as a column. 

Ask:
The field to be imported is available in the dropdown to map and hence we should import that field like we are doing it with External system import > csv.

Workaround:
Make sure that the field is available on the issue screen before importing it.